The field of this invention relates to the controlled release of agricultural chemicals to their target organisms. Materials of particular interest are pesticides and fertilizers which are normally applied directly to the soil, to the plant foliage, or to an inert carrier.
Upon application, many biologically active materials, such as pesticides, rapidly lose their effectiveness. They are subject to run-off, spray-drift, volatilization, leaching, sorption by organic matter, and chemical, photochemical, and microbiological degradation. To compensate for the loss of pesticide activity, applicators tend to apply dosage rates in excess of the prescribed concentration. This creates an added expense and also an environmental hazard, particularly to nontarget animal and plant life.
Attempts have been made to produce sustained release compositions by physically mixing pesticides with various carrier substances, such as kaolin clay. Other compositions have employed wetting agents, surfactants, fillers, and colloids to provide a coating to enhance sticking to the surfaces of the active sites. These compositions have not been satisfactory in that the additives frequently alter the toxicity of the active ingredient, and are not always stable under adverse weather conditions.
Other pesticide systems have used synthetic polymers as encapsulating agents or carriers, such as described in U.S. Pat. Nos. 3,851,053 and 3,660,563. However, these polymers have not shown clear metabolic rates in soil and are under attack by the Environmental Protection Agency and the Food and Drug Administration. Moreover, encapsulating materials for pesticides must be formulated to permit constant rate diffusion through surface pores therein. Fabrication of such devices is difficult and expensive.
Another problem exists in putting the active ingredients in sprayable form at the desired concentration. As pointed out in U.S. Pat. No. 2,390,941, most herbicides, for example, are insoluble in water. The patent further mentions that prior attempts to dissolve the insoluble compounds in suitable organic solvents and to subsequently dilute them with water have resulted in precipitation of the compounds.
U.S. Pat. No. 3,813,236 discloses a method of chemically combining organic pesticides with a natural polymeric substrate such as lignin or a lignin-containing material. The product is an insoluble composition which suffers from the limitation of not being in a conveniently sprayable form as discussed above.
A sprayable pesticide composition which overcomes some of the problems in the prior art is taught in U.S. Pat. No. 3,717,452. The procedure disclosed therein involves incorporating a small amount of Xanthomonas hydrophilic colloid in the aqueous carriers for the agricultural chemicals. The viscosity of the mixture imparted by the colloid retards spray-drift, run-off, and evaporation. However, the water solubility of the colloid mixture renders it susceptible to washing by rain, thereby limiting the effectiveness of the active ingredient. Moreover, the economic attractiveness is diminished by the expense of producing and recovering the Xanthomonas colloid.